1. Field of the Invention
This invention relates to an apparatus and method for determining the stiffness of sheet material such as bank notes.
2. Discussion of Related Art
Such an apparatus is known from EP-A 0 073 133. In this apparatus the sheet material is guided between two flat belts in the middle and deflected via a double-conical roller by an angle of 180.degree.. Simultaneously the sheet material is deformed in the longitudinal and transverse directions. The sounds produced during this process are detected by a microphone. From the detected sounds an evaluating device determines the stiffness of the sheet material.
A disadvantage of the apparatus is that the produced sounds decrease greatly when the stiffness of a sheet is measured several times. The stiffness of the sheet material lessens with each measuring process due to the deformation involved in the measuring process.
Starting out from this, the invention is based on the problem of proposing an apparatus and method for determining the stiffness of sheet material where the stiffness of the sheet material remains essentially unchanged.